This invention relates to a propelling nozzle for an aircraft engine and, more particularly, to a propelling nozzle for an aircraft engine which has an interior shroud of high thermal conductivity penetrated by a number of cooling ducts and surrounded from the outside by a solid support shroud.
The previously known propelling nozzles which are used, for example, in rocket engines for carrier rockets or in the space shuttle have a rotationally symmetrical contour. In particular, the circular cross-section tapers from the combustion chamber in the direction of the narrow cross-section in order to subsequently again widen. This type of a rotationally symmetrical contour is very simple with respect to production techniques and permits an effective absorption of the gas forces.
However, because of the high temperature experienced of approximately 3,000.degree. C., the propelling nozzle must be efficiently cooled. In this type of propelling nozzle, this is achieved by the construction which normally comprises an interior shroud consisting of a copper alloy in which cooling ducts are embedded in the circumferential direction or in the axial direction. These cooling ducts are cooled by a cooling medium, preferably the liquid hydrogen which is to be burnt in the propelling nozzle. On the outside, this interior shroud is surrounded without joints by a support shroud which absorbs the gas pressure forces. This support shroud must have a tensile strength that is as high as possible, while at the same time, because of the cooling arrangement disposed in the interior, the heat resistance is not very important.
Endeavors are currently being made to develop so-called hypersonic airplanes which also have a propelling nozzle of this type. The problem in the case of propelling nozzles for these airplanes is the required high efficiency during the thrust generation in which case several engines must be arranged next to one another. In order to meet these requirements, propelling nozzles are suggested having cross-sectional contour changes from the round cross-section in the area of the combustion chamber to the rectangular cross-section in the area of the nozzle outlet or even of the narrow cross-section of the nozzle.
This in turn means that the nozzle wall must have a design which is curved in a complicated manner. On the one hand, the relatively soft interior shroud must have an accurately shaped inside contour in order to achieve an optimal mass flow. On the other hand, the support shroud for reasons of stability, must be form-rigid to the extent that an adaptation to the shape of the interior shroud is not possible. However, with respect to production techniques, the manufacturing of the two shrouds with such a high accuracy of shape requires high expenditures because of the complicated geometry.
Nevertheless, it cannot be reliably excluded that, after the joining together of the two shrouds, hollow spaces may remain which during operation may result in deformations and cracks, and therefore in failures.
Based on the above, there is needed a propelling nozzle of this type as well as a process for manufacturing such a propelling nozzle which, while the production expenditures are low, permits the construction of a nozzle wall with high accuracy with respect to shape. At the same time, it is ensured that no hollow spaces remain between the two shrouds.
According to the present invention, these needs are met by a propelling nozzle for an aircraft engine which has an interior shroud of high thermal conductivity. The interior shroud is penetrated by a number of cooling ducts and is surrounded from the outside by a solid support shroud. A cast-in intermediate layer is provided between the interior shroud and the support shroud.
The principal advantages of the invention are that the manufacturing tolerances of the interior shroud and of the support shroud may be compensated. Instead of producing the two contours of the interior shroud and the interior contour of the support shroud with high expenditures and with high accuracy with respect to shape and, in addition, monitoring the joining of these shrouds with high testing expenditures only the interior contour of the interior shroud must be produced accurately with respect to shape. Concerning the other contours, very low requirements are advantageously sufficient. It is also an advantage that the surface quality of all surfaces does not exercise any influence except for the interior nozzle surface which normally also requires considerable working expenditures.
Finally, sensors, such as temperature measuring probes and pressure measuring probes, can be let into the nozzle shroud without weakening the support shroud. The sensors are cast into the intermediate layer. With low expenditures, all spaces between the interior shroud and the support shroud, including undercuts, can be filled in completely.
The whole shroud surface is supported by the support shroud in a manner which is accurate with respect to the measurements. Targeted characteristics of the shroud can be produced by the selection of a suitable material for the intermediate layer. Thus, it is possible to generate increased ductility or prestress, particularly when a material is chosen which expands in a targeted manner during solidification.
The intermediate layer preferably consists of a metal alloy, particularly an alloy in which bismuth and/or tin is the main constituent. These alloys have relatively low melting points and thus permit a casting-in with relatively low expenditures. Since the hydrogen cooling results in a very high cooling effect, the temperatures in the area of the intermediate layer may be kept very low so that there is no risk of melting during the operation. For example, tin - copper alloys may be used which have a melting point in the range of 220.degree. C. Bismuth alloys have an even lower melting point. As an alternative, the use of cadmium alloys can result in the setting of a higher melting point in the range of approximately 300.degree. C. Naturally, the use of higher-melting metals, such as copper, is also possible. However, in this case, increased precaution must be taken against the risk of thermal warping of the two shrouds during the casting-in. According to the selection of the constituents contained in the alloy, a defined solidification behavior can be established, i.e., an expansion for producing a specific prestress or no expansion for achieving an arrangement that is free of tension.
As an alternative, it is also possible to produce the intermediate layers from different materials, for example, from a ceramic mass in the form of neutralized silicic acid. Finally, the use of plastic materials for the intermediate layer is also advantageous, such as temperature-resistant two-constituent bonding agents.
As an advantageous further development of the invention, the intermediate layer has a thickness of 0.5-5 mm, preferably approximately 1 mm. As a result, it is ensured that the intermediate-layer material can be poured in well and with a low consumption of material.
Another advantage connected with the invention is the fact that the intermediate layer can be removed again by being melted out, and the interior shroud may be exchanged without changing the support shroud. The support or pressure housing may advantageously be manufactured from a high-strength and weight-reducing fiber composite such as carbon fiber/plastic, and carbon fiber/graphite.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.